The present invention relates to power amplifiers and, in particular, to the detection of a bias level in power amplifiers.
In conventional wireless devices, power amplifiers in a given frequency band for a given communication standard are typically optimized to meet specifications for output power, gain, linearity, and attain a maximum efficiency given the previous constraints. The control of direct current (DC) quiescent current levels in power amplifiers for wireless devices may effect the control of gain, linearity, and low power efficiency in handset application. In addition, in Global Standard for Mobile Communications (GSM) applications, under radio frequency (RF) drive conditions the linear relationship between DC collector current and output RF voltage amplitude in saturated bipolar power amplifiers has been exploited by current sensing the collector current dynamically. Extensions of the use of saturated bipolar-based power amplifiers in architectures that enable linear amplification also may utilize current sensing of the DC current.
As seen in FIG. 1, conventionally, a series resistor 26 between the power supply and the power amplifier (PA) 20 detects the DC current levels. As is seen in FIG. 1, the DC voltage across the sense resistor 26 may be measured by operational amplifier 22 and fed back to a DC adjust circuit 24 which controls the DC bias of the PA 20. Inductor 28 may filter any alternating current (AC) component from the voltage and inductor 30 may be provided for matching purposes.
When a series sense resistor 26 is utilized to measure DC current levels, the sense resistor 26 may lead to dissipated power depending on the resistor value. Furthermore, the sense resistor 26 typically is a high accuracy low value resistor to preserve efficiency which may be expensive. For small resistor values, the degradation in efficiency may be small and can be quantified according to:
xcex7w/Res/xcex7w/oRes=1xe2x88x92IDCRSENSE/VDD
where is the total IDC current, RSENSE is the value of the sense resistor between the PA 20 and the power supply (typically a battery in a wireless device), and VDD is the power supply voltage. For a typical application of a 1 Watt RF output and 50% efficiency, with RSENSE=0.1 Ohm, and VDD=3.4 V the degradation in efficiency will be 2%. For a larger, 1 Ohm, resistor, the efficiency degradation is closer to 18%.
In addition to the incremental loss in peak efficiency, the accuracy of the current sensing system may be sensitive to the exact value of the very small resistance. Such high accuracy, extremely small resistance values may be costly and/or difficult to manufacture.
It is the inventor""s belief that previously a single transistor from a multi-transistor power amplifier was used to measure DC current for the amplifier by providing a separate power feed to the single transistor.
Embodiments of the present invention provide methods and systems for current sensing for an amplifier using an embedded cell. The embedded cell is a transistor cell from a plurality of transistor cells which is coupled to the other transistor cells so as to block DC current flow between the embedded cell and the other cells and allow AC current to flow between the embedded cell and the other cells. Power may be supplied to the embedded cell through a current sensing circuit, such as a resistor, which senses the DC current drawn by the embedded cell which reflects to the total DC current drawn by the by amplifier.